Cold cathode pressure sensors are Penning, magnetron or inverted magnetron pressure measuring cells, and are primarily used for measuring a gas pressure and/or a partial gas pressure in the high vacuum range. A cold cathode pressure sensor comprises a gastight housing having a test gas inlet through which the test gas enters into the housing. In the housing a cold cathode discharge is induced between an anode and a cathode with a high voltage being applied, wherein an ion current in the burning discharge is nearly proportional to the test gas particle density in the housing. During the cold cathode discharge numerous small electron ring currents flow in the anode region. The ring current electrons collide with gas molecules in the housing. The electron surge causes an ionization of the gas molecules and atoms such that they fly to the cathode and receive another electron there. The voltage pulses thus generated serve as a measure for the particle density, i.e. the test gas pressure in the housing.
When the electron surge is ionized in the housing, and the generated ions impinge on the cathode, secondary electrons are produced which feed the ring current in the housing. With an increasing electron density, the loss rate of the ring currents increases, too, due to the Coulomb repulsion such that a constant ring current is produced in the housing. The buildup of a constant ring current normally takes place within a split second, and is referred to as “ignition”.
At a very low gas pressure in the housing, the probability of a surge occurring between an electron and a gas particle may be so small that the generated secondary electrons are not capable of compensating for the electron losses. In this case, a constant ring current is not build up until the number of primary electrons in the housing happens to be sufficient for triggering the ignition.
For this reason a radiation source is provided and/or used for igniting the Penning discharge. From U.S. Pat. No. 5,157,333 a cold cathode pressures sensor is known, wherein in the housing a radiation source is arranged which starts the discharge. The radiation source generates photons which release electrons from the metal cathode due to the photo effect. In metals the photoelectric exit energy for electrons amounts to several electron volts, e.g. for the frequently used cathode metal titanium 4.33 eV. For inducing the photo effect, the ignition radiation must therefore have a wavelength of 287 nm which corresponds to UV light the generation of which is a complex process. The UV radiation source is arranged inside the housing since there are no suitable inexpensive housing materials which are sufficiently permeable to UV light and yet gastight. However, arrangement of the radiation source inside the housing is relatively complicated since electric conduits must pass through the housing wall, and cooling of the radiation source is virtually impossible.